1. Field of the Invention
The invention relates to a pumping system and an associated method for the pumping of an effluent from a sub-sea well where the effluent is transported to a floating surface platform or to an on shore site for processing. More particularly, the invention relates to a pressure compensator for a sub-sea pumping station for pumping a multiphase effluent and which is used in conjunction with a deep-sea well head.
2. Background Information
As shallow offshore oil and gas production well reservoirs are being depleted, more nations and/or companies are taking a greater interest in deep-sea offshore oil and gas reservoirs in which sub-sea multiphase pumping systems are used to extract and pump the oil and/or gas from these reservoirs.
A sub-sea multiphase pumping system transports a multiphase effluent, which generally consists of mixtures of oil, gas, and water, from a sub-sea pumping station over a long distance through a pipeline to a remotely located processing plant where the multiphase effluent is then separated into individual fluid components prior to further processing. This processing plant may be on a floating surface platform or may be on the land.
Worldwide, several different types of sub-sea multiphase pumping systems are currently being developed and each type of multiphase pumping system consists of the same basic components: a multiphase pump, a drive for the multiphase pump, a power supply system, a control system, a pressure maintenance system, and auxiliary lubricating and cooling circuits for the multiphase pump/drive unit. A sub-sea multiphase pumping system generally consists of one or more of these basic components which are mounted on a base and then lowered and installed onto sub-sea trees where they are connected to a deep-sea wellhead.
The types of pumps in use today in the multiphase pumping system are either a rotodynamic pump or a positive displacement pump as these types of pumps are generally able to handle more than one effluent phase. In the deeper sea depths, the latter type of pump is used in that it is less sensitive to density and, therefore, less sensitive to the pressure variations of the multiphase effluent being pumped. Nevertheless, the sub-sea multiphase pump is required to maintain or increase the production rate of the multiphase effluent regardless of whether the well pressure is high or low.
The drive for the multiphase pump may be a hydraulic turbine or a variable speed electric motor, the latter having been determined to be more power efficient, more flexible in operation, and less sensitive to its remoteness from the power source.
For a hydraulic turbine, either pressurized water or oil is used as a drive fluid. The system for the pressurized water or oil is located on the floating surface platform, and several conduit feed lines are connected from this pressurized system to the sub-sea unit. Additionally, a barrier fluid system, which is generally different than the process and turbine fluids, is provided for cooling and for lubricating the bearings in the multiphase pump/drive unit and for compensating for the varying pressures in the system. This barrier fluid is routed to the floating surface platform where it is cooled and then returned to the sub-sea unit, and is maintained from the topside platform at a pressure greater than that of the process fluid so that any leakage that occurs will be of the barrier fluid either into the sea or through the mechanical seals into the process fluid.
If pressurized water is used to drive the hydraulic turbine, then the shaft seals between. the turbine and the multiphase pump can be eliminated allowing the water in the turbine to flow through the close-clearance axial gaps in the shafting between the turbine and the multiphase pump and into the production or process fluid which, as discussed above, is the multiphase effluent being pumped. In this application, the barrier fluid may also be water which circulates through the multiphase pump and through the turbine housing. Pressure compensation occurs in that the barrier fluid leakage from the turbine flows into the multiphase pump and into the process or production fluid in the pump and finally into the seawater. The barrier fluid in effect provides a backpressure to the lubricating side of the seals to insure that the leakage is into the process fluid or into the turbine fluid side of the seals.
If oil is used to drive the hydraulic turbine, then seals are used to separate the compartment for the turbine fluid from that of the multiphase effluent being pumped. Generally, oil is also used as the barrier fluid for cooling and lubricating the bearings in the multiphase pump/drive unit and for compensating for the varying pressures in the inlet of the multiphase pump. Even though the barrier fluid is compatible with both the fluid in the turbine and the multiphase effluent being pumped, one of the disadvantages of this system is that small amounts of oil tend to leak into the surrounding seawater thereby creating an environmental problem.
Even though the hydraulic turbine multiphase pumping systems are considered by some as being mechanically and hydraulically simple in design and simple to maintain, the topside facilities for these types of pumping systems are required to support extensive systems for the power source, the hydraulic source, and the barrier fluid system.
The problem with these facilities is that their power consumption increases dramatically with increased pressure drop as the umbilical feed lines lengthen. That is, as the sub-sea stations go deeper and are located farther from their floating surface platform, the hydraulic line losses for the hydraulically turbine driven multiphase pump increases. In general, the more removed the energy source is from the sub-sea station, the more complex the recirculating umbilical feed lines and, therefore, the more costly it is to provide this type of boosting system for extracting a multiphase effluent from the deep-sea well.
Some system designers have recognized that for deeper wells, submerged motors provide a more economical alternative to the hydraulic turbine drive. In one such system, an electro-submersible pump has its motor, and in some applications, a transformer located on the sub-sea station. The motor/pump unit can both be oil cooled, or the motor can be water cooled and the pump can be oil cooled. In the first system where the oil is the sole lubricating and cooling agent, the oil system also provides the pressurization of the system to prevent back leakage of fluid from the pumped fluid, and the oil is transported to an air cooled cooling unit on the floating surface platform. Even though this system is the simpler of the electrical driven systems, it still requires umbilical feed and return lines which recirculate the cooling medium to the cooling unit on the floating surface platform and back to the sub-sea station.
In the second system where the motor is water-cooled and the pump is oil-cooled, there is an oil cooling circuit for the multiphase pump bearings and seals, and a water-glycol circuit for the submerged electric motor bearings and seals. The shaft seal leakage from each lubricating circuit enters a chamber between the motor and pump which houses the coupling for the motor and pump. The oil and water-glycol mix is collected in a leak-off tank. The water-glycol and oil solutions are periodically pumped to the floating surface platform where they are separated and recycled back to their respective sub-sea supply tanks. Each of the supply tanks have a bladder-type diaphragm which communicates with the oil supply tank, which, in turn, is in communication with the pump suction and which, therefore, regulates the pressure in the other tanks, resulting in all three tank pressures being equalized to the pump suction pressure during all modes of operation of the system regardless of the external pressure and water depth. A sub-sea heat exchanger for the oil and a sub-sea heat exchanger for the water-glycol transfer their heat loads to the surrounding water, and auxiliary impellers attached to the main drive train circulate the two coolant fluids through the motor and the pump whenever the motor is running. The umbilical connections between the sub-sea station and the floating surface platform comprise a three-phase electrical feed, a makeup oil line to the oil supply tank, a makeup water-glycol line to the water-glycol supply tank, and a leak-off line to the oil/water-glycol separator unit resulting in an increase in the size of the umbilical connections and therefore, a complex design for this two fluid system.
In general, the current technologies which feature sub-sea motors employ wet winding motors whose windings are directly cooled by the hydraulic cooling circuit medium which generally is oil. A disadvantage of using a wet winding motor is that the direct contact of the windings with the coolant reduces the long-term reliability of the motor even though special insulating materials are being used. Failure of the motor results in a substantial loss of production and increased maintenance costs in that in order to resume operation, the sub-station must be removed and replaced.
For a deep-sea well there is a continuing interest in submerged electrical motor driven pumps for the pumping of an effluent, which may be a multiphase effluent. However, the present system designs are costly and complex, and require a great degree of maintenance and manned topside support for their operation.
There remains, therefore, a need in the art to simplify the design for a sub-sea single or multiphase pumping system, to decrease the costs involved in providing a sub-sea single or multiphase pumping system, and to provide a more technically superior and economically advantageous single or multiphase pumping system.
There is a further need to provide a sub-sea single or multiphase pumping system which is substantially maintenance-free, requiring very little or no human intervention for its operation, and which has an increased life expectancy compared to present-day systems.
There is also a need for a sub-sea pressure compensating system for use in conjunction with a sub-sea single or multiphase pumping system which remotely controls pressure levels of the system.